Highly uniaxially oriented films have generally been produced by uniaxially stretching a melt extrusion film or solution cast film at a temperature of from the glass transition temperature to the melting point, or by uniaxially stretching in a solvent capable of swelling the film to an appropriate degree. The uniaxially oriented films obtained by these orientation processes are liable to suffer from pinholes or strain from the nature of the production steps. In order to relieve the strain, heat treatment is sometimes performed. However, heat treatment is not always sufficient for relieving the strain, or impairs the flatness of the films, or causes optical nonuniformity or crystallization only to make the film turbid white.
Japanese patent application (OPI) No. 102205/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a process for producing polarizing films, in which a monomer having liquid crystal properties and the formula: ##STR1## wherein R represents, for example ##STR2## etc., is interposed between Nesa glass plates having been subjected to a rubbing treatment, and high voltage is applied to the glass cell to cause orientation and polymerization of the monomer.
However, this process has many problems such as: expensive Nesa glass should be used; application of high voltage is required; the temperature region in which the monomer shows a liquid crystal state is high; unevenness due to shrinkage is generated upon polymerization; and the like. Further, in cases when the above-described monomer is thermally polymerized in the presence of a polymerization initiator, such as AIBN (azobisisobutyronitrile), it is difficult to control the polymerization initiating temperature. Furthermore, in order to achieve rapid polymerization and also in view of bulk polymerization, orientation and polymerization should be carried out at a high temperature, usually above 100.degree. C., making the production step very troublesome. Measures for solving these problems include photopolymerization. According to the photopolymerization process, polymerization can be effected at a temperature freely selected from the range of from room temperature to about 100.degree. C. Therefore, monomers displaying a liquid crystal state (hereinafter referred to as liquid crystal monomers) at a relatively low temperature can be used, thus markedly simplifying the film production step.
Nevertheless, photopolymerization requires addition of a photopolymerization initiator, which, in some case, may result in another problem in that the liquid crystal region of the liquid crystal monomer becomes narrow depending upon the kind of photopolymerization initiator used, resulting in a failure to obtain a stable orientation state.